Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
In one form, a guided wave radar (GWR) transmitter uses a coaxial probe that functions as an electrical transmission line into the process vessel. The GWR measurement process begins with an electrical pulse that is launched along the probe from one end. A typical coaxial probe 10 is illustrated in FIG. 3. A TDR circuit identifies impedance discontinuities along the length of the probe, as shown in the impedance chart of FIG. 3. One source of an impedance discontinuity occurs at the vapor to liquid interface due to the difference in the relative dielectrics of the media. The TDR circuit detects, and locates in time, the reflected signal from the interface. Another source of an impedance discontinuity can be a change in geometry in the transmission line. This is a convenient method for producing a known reference location, called a fiducial (FID) in the probe. The difference in the TDR time measurements of the fiducial to the vapor to liquid interface is used to calculate the liquid level. Another impedance discontinuity exists at the end of the probe (EOP). With this type of probe and TDR circuit an increased impedance creates a positive reflected signal and a decrease in impedance creates a negative reflected signal, as shown in the echo curve of FIG. 3. As is apparent, the probe, impedance chart and echo curve in FIG. 3 are aligned to illustrate physically along the probe where the impedance changes occur and the resultant echo curve caused by these impedance changes.
The velocity of the signal propagation is a function of the relative dielectric of the medium. A problem occurs when the relative dielectric of the vapor varies due to changes in temperature, pressure or vapor composition. A known solution to this problem is to create an impedance discontinuity at a known location in the process vapor region, called a reference target. The reference target is used to measure the actual propagation velocity in the vapor. The measured propagation velocity is used for a more accurate level measurement. This technique is illustrated in FIG. 4 which shows a probe 12 including a target sleeve 14 on the probe center conductor 16. There are impedance changes which occur at each end of the target sleeve 14, as illustrated. This produces a negative reflection at the leading edge and a positive reflection at the trailing edge of the reference target. This results in transmission losses, which results in a smaller level signal. Also, the negative reflection can be confused as a level signal.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.